BACKGROUND: Brucellosis is one of the most common zoonosis in Middle East and Iran. OBJECTIVES: The purpose of this study was genomic detection of Brucella spp. in sero-positive dairy cattle. METHODS: We have collected 28,519 blood samples from cows during 2012-2013. Samples were screened by Slide and tube agglutination and 2-Mercaptoethanol tests. Samples with anti-Brucella antibodies titer ≥ 1:80 and ≥1:40 in tube agglutination and 2-ME tests were considered as positive respectively. Tissue samples include: lymph nodes, liver, testicle and kidney from 122 samples of slaughtered cows were collected. The Sero-positive samples were examined by a collection of specific primers for Brucella abortus, Brucella melitensis, vaccinal strains included RB51 and Rev1 using PCR tests. RESULTS: Results showed that 450 samples were positive in slide agglutination test and 447 samples had anti-brucella antibodies titer equal to or more than 1:80. So they were positive by tube agglutination test. Three hundred eighty nine samples were positive by 2- mercaptoethanol test. PCR test results showed that 46 samples (37.7%) out of 122 samples had a specific sequence of Brucella or otherwise they have an active infection with Brucella species, whereas 62.3% of samples were negative. The PCR results showed that 2 samples (4.35%) were infected by B. melitensis, 2 samples (4.35%) infected by Rev1 strain and 42 samples (91.3%) were infected by B. abortus. CONCLUSIONS: The results showed that, as we had expected, the majority of cows were infected by B. abortus. Animals who infected by B. melitensis and Rev1 strain may be a result of contact with sheep or goats. We couldn’t find Brucella genome in 76 samples (62.3%) of sero-positive cows. It may be caused by cross reaction of sera with Brucella species in tests or activation of immune system response and elimination of organism from internal organs.